Initiation of granulopoiesis in zebrafish embryos.

(A-D) Immature heterophil granulocytes in a 48-hpf zebrafish embryo. Panels A and C are low-power electron microscope views of immature heterophil granulocytes in axial tissue (A) and in an axial vessel (C). The heterophils boxed in panels A and C are recognizable by the characteristic cigar-shaped, electron-dense granules in cytoplasm and are shown at higher power in panels B and D. Scale bars = 2 μm (A), 1 μm (B).